A.pm

use v6;

class A {
  use B;
  has B $.b;

  method run {
    $.b.dispatch(self);
  }
}

B.pm

use v6;

class A { ... }
class B {
  method dispatch (A $a) {
    say $a;
  }
}

output.txt

leedo@zaphod:~/src$ perl6 -I. test.pl 
===SORRY!=== Error while compiling /home/leedo/src/B.pm
The following packages were stubbed but not defined:
    A
at /home/leedo/src/B.pm:9
------> <BOL>⏏<EOL>
    expecting any of:
        statement end
  from :4
  from test.pl:5

test.pl

#!/usr/bin/env perl6

use v6;

use A;

my $a = A.new;
$a.run;